In recent years, the applicability of compound semiconductor devices, particularly high electron mobility transistors (HEMT) using a GaN based compound semiconductor as a main material to high-output high-frequency devices has been examined. A HEMT using a GaN compound semiconductor as a main material (hereinafter, referred to as a GaN type HEMT) includes a silicon nitride film formed over the surface thereof by, for example, the plasma chemical vapor deposition (CVD) for the stabilization of electric characteristics and chemical characteristics. Many traps that cause fluctuations in current called current collapses are present on the surface of GaN type HEMT and a silicon nitride film is effective in inactivating such traps.
However, it is difficult to obtain a high drain current from a conventional GaN type HEMT including the above structure.
Japanese Laid-open Patent Publication No. 2008-205392 is known as a related technology.